Inflammation
Inflammation is a complex biological process of the body in response to an injury or abnormal stimulation caused by a physical, chemical, or biological stimulus. Inflammation is a protective process by which the body attempts to remove the injury or stimulus and begins to heal affected tissue in the body.
The inflammatory response to injury or stimulus is characterized by clinical signs of increased redness (rubor), temperature (calor), swelling (tumor), pain (dolor) and/or loss of function (functio laesa) in a tissue. Increased redness and temperature is caused by vasodilation leading to increased blood supply at core body temperature to the inflamed tissue site. Swelling is caused by vascular permeability and accumulation of protein and fluid at the inflamed tissue site. Pain is due to the release of chemicals (e.g. bradykinin) at the inflamed tissue site that stimulate nerve endings. Loss of function may be due to several causes.
Inflammation is now recognized as a type of non-specific immune response to an injury or stimulus. The inflammatory response has a cellular component and an exudative component. In the cellular component, resident macrophages at the site of injury or stimulus initiate the inflammatory response by releasing inflammatory mediators such as TNFalpha, IFNalpha IL-1, IL-6, IL12, IL-18 and others. Leukocytes are then recruited to move into the inflamed tissue area and perform various functions such as release of additional cellular mediators, phagocytosis, release of enzymatic granules, and other functions. The exudative component involves the passage of plasma fluid containing proteins from blood vessels to the inflamed tissue site. Inflammatory mediators such as bradykinin, nitric oxide, and histamine cause blood vessels to become dilated, slow the blood flow in the vessels, and increase the blood vessel permeability, allowing the movement of fluid and protein into the tissue. Biochemical cascades are activated in order to propagate the inflammatory response (e.g., complement system in response to infection, fibrinolysis and coagulation systems in response to necrosis due to a burn or trauma, kinin system to sustain inflammation) (Robbins Pathologic Basis of Disease, Philadelphia, W.B Saunders Company).
Inflammation can be acute or chronic. Acute inflammation has a fairly rapid onset, quickly becomes severe, and quickly and distinctly clears after a few days to a few weeks. Chronic inflammation can begin rapidly or slowly and tends to persist for weeks, months, or years with a vague and indefinite termination. Chronic inflammation can result when an injury or stimulus, or products resulting from its presence, persists at the site of injury or stimulation and the body's immune response is not sufficient to overcome its effects.
Inflammatory responses, although generally helpful to the body to clear an injury or stimulus, can sometimes cause injury to the body. In some cases, a body's immune response inappropriately triggers an inflammatory response where there is no known injury or stimulus to the body. In these cases, categorized as autoimmune diseases, the body attacks its own tissues causing injury to its own tissues.
Hereditary Angioedema
Hereditary angioedema (HAE) is a rare inflammatory disease characterized by recurrent episodes of swelling around the head and extremities (Zuraw, B. L. N. Engl. J. Med. 359: 1027-36, 2008). Angioedema attacks occur with unpredictable frequency and are typically focused on the skin, and gastric, oropharyngeal, and laryngeal mucosas. Asphyxiation due to laryngeal swelling can result in mortality. HAE is caused by deficiency or malfunction of the serine protease inhibitor C1-INH (Kaplan, A. P. et al. J. Allergy Clin. Immunol. 109: 195-209, 2002). C1-INH is the primary inhibitor of coagulation factors 12 and 11 (Factor 11) of the intrinsic coagulation pathway as well as plasma kallikrein (Gigli, I. et al. J. Immunol. 104:574-581, 1970). C1-INH mediated inhibition of plasma kallikrein and Factor 12 results in inactivation of the kallikrein pathway and decreased levels of bradykinin (BK). C1-INH deficiency or dysfunction results in overproduction of BK, which is the mechanism by which HAE attacks are believed to occur. Type III HAE has been linked with mutations in the Factor 12 gene, which encodes coagulation protein Factor 12 (Cichon, S. et al. Am. J. Hum. Genet. 79: 1098-1104, 2006).
The kinin-kallikrein pathway consists of several proteins that play a role in inflammation, blood pressure control, coagulation, and pain. Plasma prekallikrein is the precursor of plasma kallikrein, which in turn liberates kinins from kininogens and also generates plasmin from plasminogen. Plasma prekallikrein is converted to plasma kallikrein by Factor 12a by the cleavage of an internal Arg-Ile peptide bond. Plasma prekallikrein, in turn, is the product of the KLKB1 gene (MacKenzie, J. A. et al. Appl. Physiol. Nutr. Metab. 35: 518-525, 2010. Plasma kallikrein works in association with Factors 11 and 12.
There is currently no animal model which directly replicates HAE. However, the increased vascular permeability associated with HAE has been replicated in rodent models with agents such as the angiotensin converting enzyme (ACE) inhibitor captopril, as well as the C1-INH knockout mouse (Han, E. D. et al. J. Clin. Invest. 109: 1057-1063, 2002).